This invention relates to a method of checking the rotational frequency of a spindle in a numerically controlled machine tool.
In such machining operations as turning, tapping and drilling, a spindle is rotated at a speed in accordance with a rotational frequency command from a numerical control device (referred to as an NC), whereby a workpiece or tool is caused to rotate.
In a lathe, for example, a spindle is rotated on the basis of an S-function instruction (a spindle rotational frequency instruction issued in the form of the letter of the alphabet S followed by a two-digit numerical value) from an NC, whereby a workpiece loaded on the spindle via a chuck is caused to rotate. While the workpiece is rotating, a cutter is transported in the direction of the central axis of the workpiece (the Z-direction) and in a direction orthogonal thereto (the X-direction) to subject the workpiece to a predetermined turning or thread cutting operation. In thread cutting work, threads cannot be cut at a precise pitch unless the spindle is rotated at the commanded velocity. Even in an ordinary turning operation, moreover, tool life is shortened unless the spindle is rotated at a commanded velocity, namely at the commanded rotational frequency.
In performing tapping machining by a machining center or the like, a screw tap mounted on a spindle is rotated in accordance with a commanded rotational velocity while the screw tap is transported in synchronism with the rotational velocity to cut threads in a workpiece. Accurate thread cutting cannot be carried out unless the screw tap is rotated at the commanded velocity.
From the foregoing, it can be seen that there is a need to rotate the spindle at the commanded rotational velocity at all times, or to rotate the spindle within an allowable percentage of change. If the allowable percentage of change is exceeded, then this must be sensed quickly and the problem eliminated. In the prior art, however, an effective method of checking the rotational frequency of a spindle is not available.
Accordingly, an object of the present invention is to provide a novel method of checking the rotational frequency of a spindle wherein, when the spindle rotational frequency is checked and found to exceed the allowable percentage of change, or when the actual rotational velocity does not attain a predetermined velocity even after the passage of a prescribed length of time following a change in commanded velocity, these phenomena are regarded as being abnormalities in response to which an alarm signal is produced.